The present invention relates to the field of switching power supplies. More particularly, the present invention relates to systems for and methods of performing synchronous rectification in a switching power converter.
An off-line switching power supply receives power from an alternating-current (AC) power source and provides a voltage-regulated, direct-current (DC) output that can be used to power a load. An exemplary off-line power supply includes a power factor correction (PFC) stage and a DC-to-DC converter stage. The PFC stage receives the AC input signal, performs rectification on the AC signal and maintains current drawn from the AC source substantially in phase with the AC voltage so that the power supply appears as a resistive load to the AC source. The DC-to-DC converter stage receives the output of the PFC stage and generates the voltage-regulated, DC output which can be used to power the load. The output of the PFC stage is typically at higher voltage and is more loosely regulated than the output of the DC-to-DC stage.
A flyback power converter (or, more simply, a flyback converter) can be employed in a DC-to-DC power converter. A flyback converter employs a transformer that transfers energy from the input of the flyback converter to its output and provides electrical isolation between the input and output of the flyback converter as well as voltage multiplication. An input voltage, such as the rectified output voltage of a PFC stage, is applied across the transformer primary winding by closing a switch; as a result, a primary winding current flows and magnetic flux in the transformer increases, storing energy in the transformer. When the switch is opened, the voltage is removed and the primary winding current falls while magnetic flux drops. As a result, a current is induced in a secondary winding of the transformer. This induced current is rectified and used to charge an output capacitor. An output voltage formed across the output capacitor can be used for powering a load.
A diode can be used to rectify the current induced in the secondary winding of the transformer. However, such use of a diode introduces inefficiency due to the non-zero on-resistance of the diode when forward biased.
What are needed are techniques for a switching power supply that improve efficiency. What are further needed are for a switching power supply that improve efficiency under a variety of loading conditions.